1. Field of the Invention
The present invention relates to a rolling guide unit in which a second raceway body is made slidable relative to a first raceway body with a number of cylindrical rolls, i.e. rolling elements interposed therebetween.
2. Description of the Prior Art
In a conventional linear motion rolling guide unit, a slider (second raceway body) that slides on a track rail (first raceway body) consists of a casing, end caps fixed to both ends of the casing, end seals attached to both ends of the end caps, and an under seal provided to the underside of the slider, The linear motion rolling guide unit further includes: a number of cylindrical rolls, i.e. rolling elements (also called rollers), that travel rolling in a raceway formed between the raceway surface on the track rail and the raceway surface on the casing; and a retainer fixed to the casing to hold the cylindrical rolls in the casing.
The linear motion rolling guide unit as described above will be explained by referring to FIGS. 7 and 8. FIG. 7 is a perspective view showing one example of a conventional linear motion rolling guide unit. FIG. 8 is a cross section of FIG. 7 taken along the line A--A. The linear motion rolling guide unit shown in the figures is a four-raceway endless linear motion rolling guide unit. It has a track rail 1 with an I-shaped cross section extending in the longitudinal direction, a slider 2 straddling the track rail 1 in such a way that it can move relative to or slide on the track rail 1, and a number of cylindrical rolls 5 rollably interposed between the track rail 1 and the slider 2. The track rail 1 is formed with recessed grooves 8 in longitudinally extending both sides 18 thereof, which form raceway surfaces 9. A recessed portion 11 of the casing 4 is formed with raceway surfaces 10 at positions facing the raceway surfaces 9 on the track rail 1.
The slider 2 has a casing 4 which is formed with the recessed portion 11 so that the casing can straddle the track rail 1, and end caps 6 attached to both longitudinal ends of the casing 4. To seal the boundary between the track rail 1 and the slider 2 when the slider 2 slides on the track rail 1, the ends of the end caps 6 are each provided with an end seal (not shown) and the underside of the slider 2 with an under seal 26.
Between the upper and lower raceway surfaces 10 of the casing 4 is formed an engagement groove 15, which engages with an engagement projection 16 formed on a retainer 3. A part of the retainer 3 is loosely fitted in the recessed groove 8 of the track rail 1 and fixed to the casing 4 by bolts 17.
In the above construction of the 4-raceway endless linear motion rolling guide unit, two raceways are formed on each side of the track rail 1 by the upper and lower raceway surfaces 10 on the track rail 1 and the upper and lower raceway surfaces 10 on the casing 4. Hence, a total of four raceways are formed on both sides of the track rail 1. In these loaded roller raceways, a number of cylindrical rolls 5 roll in contact with the facing raceway surfaces 9 and 10. The casing 4 is formed with return passages 13, 14, and the end caps 6 are formed with direction changing passages 12 that connect the loaded roller raceways and the return passages 13, 14.
Examples of such a four-raceway endless linear motion rolling guide unit include those of Japanese Patent Laid-Open Nos. 175564/1989 and Japanese Patent Application Nos. 106311/1990 and 166326/1991.
As another example of the conventional rolling guide unit there is disclosed a bearing structure, which consists of a first raceway body (first raceway block) and a second raceway body (second raceway block), and a number of cylindrical rolls with a retainer, which are interposed between the first and second raceway blocks whose raceway surfaces each consist of two planes arranged in a V-shaped cross section. In this rolling guide unit, the cylindrical rolls are arranged alternately crossed so as to be able to receive loads in any directions.
As a further example of the prior art rolling guide unit, another bearing structure is disclosed. In this structure, a first raceway body and a second raceway body are provided in the form of an inner ring and an outer ring. The inner ring and the outer ring are each formed with a raceway surface which consists of two planes arranged in a V-shaped cross section. A large number of cylindrical rolls are arranged between the raceway surfaces so as to cross alternately. In this unit, the rolling surface has a line contact so that an elastic deformation caused by loads is small, allowing the bearing to support various kinds of loads such as radial load, axial load and moment load at the same time.
In the conventional linear motion rolling guide units, stress concentration usually occurs at the contact portion between the roll and the raceway, and hence crowning is applied to the rolls and raceways.
However, when the rolls are set in the raceway groove at the inclination angle of 45.degree., a large stress concentration occurs at one side of the roll. Then, if the crowning is used to increase the diameters at both ends of the roll, another problem arises that the load capacity of the guide unit as a whole virtually decreases. While it is possible to apply crowning to the end portions of the rolling surface of the roll so that the end portions have different amounts of crowning, this will result in an increased cost because of complicated assembly.
When a number of cylindrical rolls 5 travel rotating between the raceway surface 9 of the track rail 1 and the raceway surface 10 of the casing 4, the rolls 5 perform complex motion on the raceway surfaces that are inclined at 45.degree., causing stress concentration at the edge or end portion of the raceway surface 9 of the track rail 1. As a result, the raceway surface 9 may produce flaking, dents or, in the worst case, break due to edge load. Once flaking and dents due to edge load occur in the raceway surface 9 of the track rail 1, the performance of the slider 2 having the casing 4 and the end caps 6 with respect to the track rail 1 deteriorates. Particularly for track rails 1 with smaller width, the probability of occurrence of dents caused by edge load is high.
Such flaking and dents due to edge load also occur in other rolling guide units or bearings mentioned above, where a number of cylindrical rolls are interposed between the raceway surfaces of the first and second raceway bodies.